It's the first time the princes say they've ever talked about Princess Diana as a mother and it seems they're not holding back. In the trailer for the doc, which is set to air later this year, the brothers are going through a family photo album. They share a photo of Diana in a pink sweatshirt holding a young and very smiley Prince William. "We're both in this photograph," William says. "You're in the tummy."

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Williams says his mum was "really informal" in the clip and that she loved to laugh. Harry called her "one of the naughtiest parents," because, as William explains, she understood there was more to life than being a royal and staying inside those palace walls.

What is clear from this teaser, is that the fun, vivacious, and caring version of Princess Diana the public saw, wasn't that much different than the private version. In fact, the boys say she "smothered" them with love.

This August marks the 20th anniversary of Princess Diana's passing in a Paris car crash. At the time of her death, William was 15 and Harry was only 12, but despite losing their mum so young, the boys still have the fondest memories of her — that they're finally ready to share with the world.

"She was our mum. She is still our mum, you know," Harry says in the trailer. "And, of course, as a son, I would say that she was the best mum in the world."